Interferometers are precision tools that can accurately measure the position and/or the velocity of a target device. For such measurements, a measurement mirror is generally mounted on the target device. In semiconductor device manufacturing equipment, for example, the measurement mirror can be mounted on a precision stage to permit an interferometer to precisely measure the movement of the stage while the stage positions a wafer for processing.
The measurement mirror of an interferometer generally needs to be aligned with the interferometer optics that direct a measurement beam at the measurement mirror. Plane mirror interferometers use a planar measurement mirror, which provides a uniform target area so that alignment of the position of the measurement mirror is less critical than in some other types of interferometers. Alignment tolerances for the orientation of the measurement mirror in a plane mirror interferometer can also be relaxed through use of interferometer optics having a measurement beam path that includes two reflections from the measurement mirror. With two reflections, an angular error that the first reflection causes can cancel an equal but opposite angular error that the second reflection causes.
FIG. 1 illustrates an example of a known single axis plane mirror interferometer 100 having a measurement beam path including two reflections from a measurement mirror 140. Interferometer 100 includes a beam source 110, a polarizing beam splitter 120, quarter-wave plates 130 and 170, measurement mirror 140, a retroreflector 150, a reference mirror 180, and a sensor 160.
Beam source 110 produces an input beam IN that is either a monochromatic or heterodyne beam depending on the type of interferometer. Input beam IN contains two polarization components having orthogonal linear polarizations with directions respectively corresponding to beams reflected and transmitted through a beam splitter coating in polarizing beam splitter 120. FIG. 1 distinguishes the two component beams respectively using separated dashed and solid lines, but the two component beams are collinear in an actual interferometer. When input beam IN is a heterodyne beam, one polarization component beam has a first frequency f1, and the other polarization component beam has a second frequency f2, where frequencies f1 and f2 differ slightly (e.g., by a few MHz).
Input beam IN enters polarizing beam splitter 120 where a polarizing beam splitter coating reflects one component and transmits the other component. In FIG. 1, the transmitted beam is a measurement beam, but the reflected beam could alternatively be used as the measurement beam if the components of interferometer 100 are properly rearranged. The transmitted measurement beam follows a path MA through quarter-wave plate 130 to measurement mirror 140 and is reflected back along a path MA′. (Paths MA and MA′ will be collinear if measurement mirror 140 has its ideal alignment.) Passing twice through quarter-wave plate 130 effectively rotates the polarization of the measurement beam by 90°, so that the measurement beam upon reentering polarizing beam splitter 120 reflects from the beam-splitter coating. The measurement beam then enters retroreflector 150 and is reflected back into polarizing beam splitter 120 along an offset path that is parallel to the entry path.
The offset measurement beam reflects from the beam splitter coating in polarizing beam splitter 120 and follows path MB to measurement mirror 140. The measurement beam then reflects from measurement mirror 140 and returns along path MB′. The two trips through quarter-wave plate 130 along paths MB and MB′ return the measurement beam to its original linear polarization, so that the measurement beam heading along path MB′ passes through polarizing beam splitter 120 and forms part of an output beam OUT.
The component of input beam IN that is originally reflected in polarizing beam splitter 120 forms a reference beam that follows paths RA and RA′ through quarter-wave plate 170 to and from measurement mirror 180. The reference beam has its polarization changed by two passes through quarter-wave plate 170 and passes through polarizing beam splitter 120 to retroreflector 150. The reference beam returns from retroreflector 150 along an offset path RB and passes through polarizing beam splitter 120 and quarter wave plate 170 before again reflecting from reference mirror 180 to return along path RB′. The returning reference beam on path RB′ reflects from the beam splitter coating in polarizing beam splitter 120 to merge with the measurement beam and form output beam OUT.
The reference and measurement beams differ in that the measurement beam reflects twice from measurement mirror 140, which moves with the target device. The reference beam in contrast reflects twice from a fixed reference mirror 180. Movement of the target device (and therefore the measurement mirror on the target device) causes a Doppler shift in the frequency of the measurement beam at each reflection. Measurement electronics 160 measures the frequency difference between the reference and measurement beams and compares the difference to the nominal frequency difference with no Doppler shifts, e.g., to 0 Hz for a monochromatic input beam or to a few MHz for a typical heterodyne input beam. The comparison indicates the amount of Doppler shift and therefore indicates the velocity of measurement mirror 140. The measured velocity can be integrated over time to measure the movement of the target device.
Interferometer 100 is a single axis interferometer that measures the velocity or movement of measurement mirror 140 at a point halfway between the reflections of the measurement beam from measurement mirror 140. Measuring an angular movement of measurement mirror 140 requires at least one additional measurement. The additional measurement could be performed using another single axis interferometer, but a multi-axis interferometer provides a more compact implementation by using some of the same optical elements for more than one measurement.
FIG. 2 illustrates a known two-axis interferometer 200 including a polarizing beam splitter 120, quarter-wave plates 130 and 170, measurement mirror 140, and reference mirror 180, which are used for two measurement axes. Interferometer 200 also includes a beam source 210 that generates a pair of input beams IN1 and IN2, each having the same properties as described above for input beam IN in FIG. 1. Such input beams can be generated using a beam source 110, such as described above, with beam splitter optics 212.
Polarizing beam splitter 120 splits input beam IN1 according to polarization into measurement and reference beams for a first measurement axis. The measurement beam split from input beam IN1 follows a path including paths M1A and M1A′, an offset reflection from a retroreflector 251, and paths M1B and M1B′ to form part of an output beam OUT1. Similarly, the reference beam split from input beam IN1 follows a path including paths R1A and R1A′, an offset reflection from retroreflector 251, and paths R1B and R1B′ before forming a part of output beam OUT1. Measurement electronics 261 then determines the velocity or movement of a point between the reflections where paths M1A and M1B hit measurement mirror 140.
In a similar manner, polarizing beam splitter 120 splits input beam IN2 according to polarization into measurement and reference beams for a second measurement axis. The measurement beam split from input beam IN2 follows a path including paths M2A and M2A′, an offset reflection from a retroreflector 252, and paths M2B and M2B′ to form a part of an output beam OUT2. The reference beam split from input beam IN2 follows a path including paths R2A and R2A′, an offset reflection from retroreflector 252, and paths R2B and R2B′ before forming a part of output beam OUT2. Measurement electronics 262 then determines the velocity or movement of a point between reflection points where paths M2A and M2B hit measurement mirror 140.
The two measurements obtained permit a determination of the angular motion (e.g., a pitch or yaw) of measurement mirror 140. A third measurement axis could be used to determine an angular motion (e.g., a yaw or pitch) in a different plane, and in complex systems, multi-axis interferometers having three or more measurement axes are common. A concern in these interferometers is the required size of elements such as the measurement mirror 140 and polarizing beam splitter 120. Each of the measurement beams has a finite cross-section and a required separation for separate measurement of the output beams. Measurement mirror 140 is thus large enough to accommodate the area of the measurement beam at each reflection point and the required separation between the reflection points. However, a large measurement mirror is difficult to accommodate in confined spaces such as may be found in semiconductor manufacturing equipment. Polarizing beam splitter 120 must similarly be large enough to contain all of the measurement and reference paths. The fabrication of such large optical-quality elements can be expensive and difficult. Accordingly, compact architectures for multi-axis interferometers are sought.